Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory, including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), flash memory, and resistance variable memory, among others.
Memory may be volatile or non-volatile. Volatile memory requires power to maintain the information stored therein, e.g., when power to volatile memory is lost, the information stored therein is also lost. Non-volatile memory, in contrast, does not lose the information stored therein in the absence of power, e.g., non-volatile memory can retain the information stored therein even if no power is being provided to the memory. Types of volatile memory include RAM, DRAM, and SDRAM, among others. Types of non-volatile memory include ROM, flash memory, and resistance variable memory, among others.
Types of resistance variable memory include programmable conductor memory, phase change random access memory (PCRAM), and resistive random access memory (RRAM), among others. A physical layout of a PCRAM memory device may resemble that of a DRAM device, with the capacitor of the DRAM cell being replaced by a phase change material, such as Germanium-Antimony-Telluride (GST). A physical layout of an RRAM memory device may include memory cells including a variable resistor thin film, e.g., a colossal magnetoresistive material, which can be connected to an access device, such as a diode, a field effect transistor (FET), or a bipolar junction transistor (BJT), for example.
The memory cell material of a PCRAM device, e.g., GST, for a single-level cell (SLC), may exist in an amorphous, higher resistance state, or a crystalline, lower resistance state. The resistance state of the PCRAM cell may be altered by applying sources of energy to the cell, such as current pulses or pulses of light, among other sources of energy. For example, the resistance state of the PCRAM cell may be altered by heating the cell with a programming current. This results in the PCRAM cell being programmed to a particular resistance state. In a binary system, for example, the amorphous, high resistance state may correspond to a logic state of 1, and the crystalline, low resistance state may correspond to a logic state of 0. However, the choice of these corresponding logic states may be reversed, that is, in other binary systems, the amorphous, high resistance state may correspond to a logic state of 0, and the crystalline, low resistance state may correspond to a logic state of 1. The resistance state of an RRAM cell, e.g., the variable resistor thin film, may be increased and/or decreased by applying positive and/or negative electrical pulses across the film. This may result in the RRAM cell being programmed to a particular resistance state.
An SLC may represent two programmed states as represented by the binary digits 1 or 0. Memory cells may also be programmed to more than two states, such as to a number of states that allows a cell to represent more than two binary digits, e.g., 1111, 0111, 0011, 1011, 1001, 0001, 0101, 1101, 1100, 0100, 0000, 1000, 1010, 0010, 0110, and 1110. Such cells may be referred to as multi state memory cells, multibit cells, or multilevel cells (MLCs). The memory cell material of a PCRAM device including MLCs may exist in a number of intermediate states between what are generally considered to be amorphous and crystalline. MLCs may allow the manufacture of higher density memories without increasing the number of memory cells since each cell can represent more than one digit, e.g., more than one bit.